City Zones There is no connection from Airspace to Courtyard for attackers. The garrison consists of 3 zones

Each city is split into different City Zones. Possible zones include Airspace, Outer walls, Tunnels and Garrison. Not all cities have all types of city zones, and some may have some that aren't listed. However, each city must have a Garrison Zone. Furthermore, each City Zone may be split into different sub-zones.

Contents

Airspace

Airspace can only be entered by Flying units. Airspace can attack, or be attacked by, Tower garrison sub-zone.

Outer Walls

Outer Walls is a city zone that borders the Courtyard garrison sub-zone. Outer Walls can only be entered once the walls have been breached. The more units on the wall the harder it is to breach. Erf-b1-p098a

Sub Zones

Outer Walls can contain a Gate sub-zone. Gates, while weaker than other sections of the wall, allow non-flying, non-tunnel capable units to enter and exit a city.

Outer walls can also contain towers that boost its defensive value. It has yet to be seen if outer wall towers are a true sub-zone or an enhancement providing addition.

Sub-Zones

Each sub-zone could be attacked from only one of the main city zones. Outer Walls only connect to Courtyard, Airspace connects to Tower, and Tunnels connect to Dungeon. Once an attacker enters a Garrison zone, it can assault any other garrison sub-zone.Erf-b1-p116a

Speculation

Each city may have different Garrison sub-zones, depending on it's construction. For instance, a subterranean city wouldn't have an airspace and therefore, no tower. Similarly, not each garrison sub zone needs to connect to main city zone. A subterranean city might have a courtyard, but no outer walls. Spacerock is one such oddity, having dungeons but no tunnels (this is marked up to being the unique aspect that comes with being a Level 5 city) Erf-b2-p25.

A Courtyard possibly provides a bonus to certain units on defense, such as Warlords and/or pikers